This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to 99-18853 filed in Korea on May 25, 1999; the entire content of which is hereby incorporated by Reference.
1. Field of the Invention
The present invention relates to an apparatus for acquiring a pseudo noise (PN) code and a direct sequence code division multiple access (DS-CDMA) receiver including the same.
2. Description of the Related Art
CDMA is a type of spread-spectrum communication method, and is used for the interim standard (IS)-95 system, which is currently the mobile communication standard of Korea. Active research on CDMA is being currently made as the CDMA has been proposed for the international mobile telecommunication (IMT)-2000.
The CDMA system is popular since it has various advantages compared with other systems. In the CDMA system, it is not possible to distinguish noise from a transmission signal in a channel since the signal is spread using a PN code. Therefore, interception cannot be performed unless a correct code is known. Also, the CDMA system is resistant to intentional jamming due to the characteristic of the spread-spectrum system. A diversity effect is obtained in the CDMA system since a RAKE receiver is used in a multi-path channel.
The spread-spectrum method adopted in the IS-95 system is a direct sequence (DS) spread-spectrum. In the DS spread-spectrum, the spectrum is spread by multiplying the PN code, which has a duration (referred to as chip duration) much smaller than symbol duration, with a signal to be transmitted. Namely, the spectrum is spread by multiplying 64 binary PN codes with one binary symbol to be transmitted. One symbol is divided into 64 chips.
A receiver despreads a received DS spread-spectrum signal and detects symbols as they were prior to being spread. In order to despread the received DS spread-spectrum signal, it is important to acquire PN code synchronization. The synchronization of the PN code is realized by two steps of code acquisition and tracking and plays a very important role in determining the performance of the system.
In DS spread-spectrum, code acquisition can be performed by various methods. In the most commonly used method, the receiver generates the PN code using the same PN code generator as the PN code generator used in a transmitter, obtains partial correlation between the PN code and a received signal, compares the partial correlation value with a threshold value, and determines whether the code is acquired. Such a process is referred to as search.
A search range is one period of the PN code during the code acquisition. The search is performed until the phase of the PN code coincides with that of the received signal by obtaining the partial correlation value while changing the phase of the code in one period of the PN code. The one period of the PN code to be searched is referred to as an uncertainty region. In the IS-95 system, since the number of the PN code is 32768(=215), 32768 code phases are to be searched in the worst case. The uncertainty region can be divided into several sections and those sections can be searched using different correlators. The search method is divided into a serial search method, a parallel search method, and a hybrid search method according to how many correlators are to be used for searching the uncertainty region.
In the serial search method, the entire uncertainty region is searched using one correlator. In the serial search method, the code is acquired by repeating processes of obtaining a correlation value with respect to one code phase, determining whether the code is acquired, and searching another phase when the code is not acquired. In the serial search method, hardware is much less complex than in the parallel and hybrid search methods, since only one correlator is necessary. However, much more time is spent on acquiring the code than in the other methods.
In the parallel search method, the code is acquired using the correlator in parallel with respect to the entire uncertainty region. In the parallel search method, much less time is spent on acquiring the code than in the serial search method, since the entire uncertainty region is searched at one time. However, since as many correlators as the number of the code are necessary, the complexity of hardware increases in proportion to the number of correlators.
In the hybrid search method where the serial search method and the parallel search method are combined, the uncertainty region is divided into several regions and is searched by several correlators. Namely, the correlators of the serial search method, where the regions to be searched are reduced, search the uncertainty regions in parallel. Therefore, in the hybrid search method, it is possible to reduce time spent on acquiring the code compared to in the serial search method and to reduce the complexity of hardware compared to in the parallel search method.
The structure of the correlator for obtaining the partial correlation value during the code acquisition is divided into an active correlator and a PN matched filter.
In the active correlator, the partial correlation value is obtained by multiplying one input data with one code generated by the PN code generator and integrating the multiplication result for an N-chip duration. That is to say, the PN code of N bits is integrated chip-by-chip.
FIG. 1 shows the structure of a hybrid searching unit in which the conventional active correlator is used. The searching unit shown in FIG. 1 includes a PN code generator 100, a plurality of multiplexers MUXs 102, 103, and 104, and a plurality of accumulators 105, 106, and 107. As shown in FIG. 1, a hybrid searching unit searches the uncertainty region by using K different PN code phases at one time. The PN code generator 100 generates K PN codes having different phases. The PN code which is a binary code has a value of 0 or 1. The MUXs 102, 103, and 104 output +dn when the PN code is 0 and output xe2x88x92dn when the PN code is 1. Namely, the MUXs 102, 103, and 104 output dn when the PN code is 0 and output xe2x88x92dn when the PN code is 1 from input data dn received at a chip rate and the PN codes generated by the PN code generator 100. The output data of the MUXs 102, 103, and 104 are accumulated by the K accumulators 105, 106, and 107 for a certain time. The accumulated values become partial correlation values S0 through SKxe2x88x921 with respect to K different PN code phases. The partial correlation values are finally compared with predetermined threshold values. Accordingly, it is determined whether the code is acquired.
However, this method has a problem in that a long time is spent on acquiring the code, since one partial correlation value is obtained with respect to the input data of N bits.
FIG. 2A shows the configuration of a hybrid searching unit in which the conventional PN matched filter is used. The hybrid searching unit shown in FIG. 2A includes N delayers 200, a first N-code storage portion 201, a second N-code storage portion 202, and a Kth N-code storage portion 203. As shown in FIG. 2A, delayers 200 delay the input data dn for the chip duration. The code storage portions 201, 202, and 203 store the PN codes in advance. The N input data output from the delayer 200 are multiplied with the respective codes stored in each code storage portion 201, 202, and 203 and the multiplication results are added to each other, thus obtaining the partial correlation values S0 through SKxe2x88x921 are obtained. The partial correlation values are finally compared with predetermined threshold values. Accordingly, it is determined whether the code is acquired.
FIG. 2B shows the PN matched filter corresponding to a (Kxe2x88x921)th code storage portion among the PN matched filters shown in FIG. 2A. The PN matched filter shown in FIG. 2B includes a (Kxe2x88x921)th code storage portion 203, a plurality of multipliers 211, 212, and 213, and an adder 214.
The multipliers 211, 212, and 213 multiply the input data of N bits dNxe2x88x921 through d0 input from the delayers 200 shown in FIG. 2A with N codes cKxe2x88x921,Nxe2x88x921 through cKxe2x88x921,0, respectively. The adder 214 adds the outputs of the multipliers 211, 212, and 213 to each other and outputs the partial correlation value SKxe2x88x921 with respect to the (Kxe2x88x921)th code phase.
Meanwhile this method has an advantage in that it is possible to reduce time spent on acquiring the code since one correlation value is obtained with respect to one input data, this method places a large burden on the hardware, since the PN codes must be previously stored and has a problem in that the performance of the searching unit is largely dependent on signal attenuation according to fading channels.
Also, when the code is acquired using the above-mentioned hybrid searching unit, the number of operations for obtaining the partial correlation values linearly increases in proportion to increases in the number K of code phases or the number of correlation sections increases.
To solve the above problem, it is an object of the present invention to provide an apparatus for acquiring a pseudo noise (PN) code by grouping K generated PN codes according to a predetermined regulation, determining one among the K PN codes to be a reference code, multiplying a received signal with the reference code in each group, adding the multiplication results to each other, and sharing the addition result when the partial correlation value between the received signal and the PN code is obtained in order to acquire the PN code included in the received signal and a direct-sequence code division multiple access (DS-CDMA) receiver including the same.
According to an aspect of the present invention, there is provided an apparatus for acquiring a pseudo noise (PN) code, comprising a PN code generator for generating K PN codes, a multiplexer for multiplying a received signal by +1 or xe2x88x921 according to the value of a PN code selected to be a reference code among K PN codes and outputting the multiplication result, 2Kxe2x88x921 accumulators for accumulating the outputs of the multiplexer according to a predetermined control signal, a selector for outputting the control signal so that the output of the multiplexer can input to one of the accumulators according to (Kxe2x88x921) code values excluding the reference code, a programmable adder for adding the outputs of the accumulators to each other according to a predetermined regulation and outputting K correlation values, and a discriminator for outputting the PN code having a correlation value larger than a predetermined value among the K correlation values.
According to another aspect of the present invention, there is provided an apparatus for acquiring a PN code, comprising a PN code generator for generating PN codes of N-bit sequences having K different phases, a reference code storage portion for storing one of the K N-bit code sequences generated by the PN code generator, N taped delay lines for delaying a received signal N times, a multiplier for multiplying the output of the Nth tapped delay lines with the PN codes stored in the reference code register, a sub-summer for grouping generated PN codes according to a predetermined regulation, adding the outputs of the multiplier in each group to each other according to the groups to which the code bits stored in the reference code register belong and obtaining 2Kxe2x88x921 sub-sums, a programmable adder for adding the outputs of the sub-summer to each other according to a predetermined regulation and outputting K correlation values, and a discriminator for outputting the PN code having a correlation value larger than a predetermined threshold value among the K correlation values.
There is provided a direct-sequence code division multiple access (DS-CDMA) receiver for searching a PN code included in a received signal and despreading and demodulating the received signal using the searched PN code, comprising a PN code generator for generating PN codes of an N-bit sequence having K different phases, a code synchronizer by code grouping for grouping K PN codes generated by the PN code generator according to a predetermined regulation, determining one of the K PN codes to be a reference code, summing the multiplications of the received signal with the reference code in each group, determining a PN code having a correlation value larger than a threshold value among K correlation values obtained by adding the summed values according to a predetermined regulation to be the PN code included in the received signal, and synchronizing the phase of the determined PN code with the phase of the received signal, a despreader for despreading the received signal using the determined PN code, and a demodulator for demodulating the despread signal.